The present invention relates generally to the packaging of integrated circuits utilizing a heat spreader to cool an integrated circuit die. More particularly it relates to an arrangement utilizing glob-topping material to protect bonding wires and to secure the heat spreader to a lead frame.
Semiconductor integrated circuits are currently mass produced for a broad range of purposes. Therefore, continual cost and quality improvements in their manufacture are valuable. Significant savings in the packaging of one integrated circuit can generate large overall cost savings when mass produced.
In high performance integrated circuits, heat dissipation can be a serious problem. To combat it, some manufacturers have introduced heat spreaders to strengthen the heat coupling from an integrated circuit die to the lead frame where heat can then pass through the leads of the lead frame to the circuit board. These heat spreaders are attached to lead frames by double-sided tape. By way of example, polyimide tape is commonly used. When the heat spreader is formed from an electrically conductive material, the tape acts as an electrical insulator to prevent the shorting of the leads of the lead frame.
A conventional process for packaging an integrated circuit using a heat spreader without a die attach pad is illustrated diagrammatically in FIGS. 1a-c. FIG. 1a shows a heat spreader 20 affixed via double-sided tape 15 to a lead frame 10. The tape 15 prevents the heat spreader 20 from shorting the leads of the lead frame 10. An integrated circuit die 25 is then attached to the heat spreader as seen in FIG. 1b. This is typically accomplished in a die bonding machine. This assembly then goes to a wire bonder where the wires 30 of FIG. 1c are electrically connected to the integrated circuit die and the lead frame.
While improving heat dissipation, the heat spreader has a serious cost drawback. The use of a tape adhesive to bond it to the lead frame is relatively expensive. If it could be eliminated, a major cost savings would be achieved.
Glob-topping is a conventional technique for protecting bonding wires which connect a die to a lead frame during manufacture. The final step in packaging an integrated circuit is to encase it in a plastic molding. Typically, molten plastic is transferred into a mold surrounding the wires, the die, the heat spreader, and a portion of the lead frame. The plastic used during the transfer mold process is relatively viscous. Therefore, the flow of plastic into the mold has been known to damage the bonding wires. This is called the wire wash problem. Some manufacturing processes apply a protective low-viscosity glue over the wires before injecting the plastic to prevent wire wash damage. This glue and the process of applying it are both called glob-topping.
FIGS. 2a-c provide a diagrammatic illustration of a packaging process using glob-topping. FIG. 2a shows a lead frame 10 electrically connected to an integrated circuit die 25 by bonding wires 30. The integrated circuit die 25 is attached to a die attach pad 27. Before encasing the integrated circuit in plastic, one applies a glob-topping material 40 as in FIG. 2b. This glob-topping freezes the wires in position. After curing of the glob-topping, the assembly is put into a mold where molten material is transferred. The glob-topping effectively protects the bonding wires during the transfer molding. FIG. 2c shows a final form of the packaged integrated circuit after encasement in the packaging material 45. In the final form only the leads of the lead frame are exposed.